1. Field of the Invention
The present invention relates to a method of cleaning an inkjet head and an inkjet recording apparatus. The present invention specifically relates to a technology for efficiently removing an ink residue and the like adhered on a surface (hereinafter also referred to as ejection face) on which an ink ejection openings of an inkjet head (hereinafter also referred to as a recording head or simply a head) are formed to clean the surface.
2. Description of the Related Art
A clean-up (cleaning) technique for a recording head which ejects an ink is a very important factor of an inkjet recording method because the method is the system in which input image data is converted to the output image using a liquid ink as a medium. Main problems in requiring the cleaning are briefly described as follows.
An ink ejection recording head directly ejects an ink through a fine nozzle (hereinafter, as such collectively referred to as an ejection opening, a liquid passage communicated therewith, and an element for generating energy utilized to eject ink unless otherwise stated) to a recording medium. Accordingly, the ejected ink hits against the recording medium and bounces back, and, in addition to the main ink involved in the recording when the ink is ejected, fine ink droplets (satellites) are ejected and drift in the atmosphere in some cases. Then, these droplets become ink mists, and, in some cases, adhere around the ink ejection opening of the recording head. Furthermore, dust drifting in the air may sometimes adhere thereto. Subsequently, the ejected main ink droplets are pulled by these attached matters, thereby the ink ejection direction is deflected, i.e., the main ink droplets are blocked from going straight in some cases.
Then, as a cleaning technique for solving this problem, an instrument called a wiping is employed to remove attached matters in the inkjet recording apparatus. The instrument wipes, at a predetermined timing, the ejection face of the recording head by means of a wiping member (wiper) made of an elastic material such as rubber.
Meanwhile, for the purpose of improving the recording density, water-resistance, light-resistance and the like of a recorded matter, an ink containing pigment components as a color material (pigment-based ink) has recently been used in many cases. The pigment-based ink is made by dispersing, in water, the color material which is originally solid by introducing a dispersant or a functional group on the surface of the pigment. Accordingly, the dried matter of the pigment ink formed by evaporating and drying the water content in the ink on the ejection face damages the ejection face seriously as compared to the dried sticky matter of a dye-based ink in which a color material itself is dissolved at a molecular level. A characteristic is also recognized that a high molecular compound used to disperse the pigment in a solvent tends to be adsorbed on the ejection face. This is a problem which occurs even in inks other than the pigment-based one in a case where a reaction liquid is added to an ink for the purpose of controlling the viscosity of the ink, improving light-resistance and for others, resulting in the presence of a high molecular compound in the ink.
To solve these problems, in Patent Documents 1 and 2, disclosed are techniques for removing an accumulated matter by applying a head liquid of nonvolatile solvent on the ejection face to reduce the wear of a wiper and dissolve the ink residue accumulated on the recording head in wiping the recording head. Moreover, the adhesion of a foreign matter to the recording head is prevented by forming a thin film of the head liquid on the recording head, and wiping easiness is improved by these. A construction in which the head liquid used in these wiping is stored in the body of a printer is employed.
In Patent Document 3, it is disclosed that wiping operations are performed on the ejection face of the head after a head liquid composed of nonvolatile solvent is applied on a wiper.
In addition, in Patent Document 4, it is disclosed that a dissolved liquid is sprayed on the ejection face, and thereby insolubilized matters adhered on the ejection face are removed using a wiper.
Furthermore, in Patent Document 5, it is disclosed that wiping operations are performed by dissolving ink residues on a head in a nonvolatile ink solvent held on a wiper.
The present inventors applied nonvolatile solvents on ejection faces by the methods disclosed in the above Patent Documents to verify the effect of the cleaning process. It was then found that some constituent materials of the recording head or some inks caused the removal of the accumulated matter by means of dissolving the ink residues or the targeted cleaning of the ejection face to be insufficiently performed, and that new problems occurred. To be more specific, it was found that the high molecular compound components in the ink residue were once dissolved by the head liquid, but that thereafter the dissolved high molecular compounds were more uniformly adhered on the surface of the recording head, causing changes in the surface properties which the recording head inherently has. In other words, this is because a thin film composed of the high molecular compounds is formed on the entire ejection face, and thereby the properties of the high molecular compounds control the surface properties of the recording head. In general, the properties of the ejection face of the recording head are specified by a form suitable for an ink to be used (whether or not to include water-repellent or hydrophilic properties and the like), and accordingly, the changes in the surface properties result in the changes in the ink ejection performance itself of the recording head.
In other words, it is possible to suppress a disadvantage of the accumulation of the ink residues by supplying the head liquid to dissolve the ink residues on the ejection face. However, the state where the dissolved matter of the ink residue by means of the head liquid is left on the ejection face cannot be said to be sufficiently cleaned. The desired surface properties of the ejection face are not maintained in such a state.
Patent Document 1: Japanese Patent Laid-Open No. 10-138503
Patent Document 2: Japanese Patent Laid-Open No. 2000-203037
Patent Document 3: Japanese Patent Laid-Open No. 10-138502
Patent Document 4: Japanese Patent Laid-Open No. 10-151759
Patent Document 5: Japanese Patent Laid-Open No. 11-254692